OLCI may offer it, or it may not.

I have a Biz (I) award YYZ-MUC-DEL with first segment on AC and second on LH. Would I be able to change to the AC YYZ-DEL nonstop, by paying the $75 fee at the airport? Both flights leave within a couple of hours of each other...

I'd be astonished if that works, but definitely try it and let us know... If you end up actually able to fly on the non-stop, make sure you submit your tickets to the AC refund site for a cash refund of excess taxes collected.

I agree with everything before the comma, though I'm uncertain about everything after.

The terms of the SDC say "No additional charges for fare difference apply for changes made on the day of travel when flights are operated by Air Canada."

I'm not sure I'd recommend wasting an agent's time with something that is clearly against the rules (getting this changed for $75).

It'd theoretically be to AC's advantage to transport on the non-stop if there was space available, and cancel the AP redemption on the LH segment.

If there was any uncertainty as to the timely operation of the outbound, there'd be zero question (but that's covered under IROPS policy)

With the Covid-19 and 737Max situations, agents may very well be doing so many changes and tweaks these days that something that's not strictly compliant with the rules, but not dis-beneficial to AC may very well fly.